In terms of time domain, if you're not certain you can get below 30 minutes, scale these numbers down. This workout should ideally take somewhere in the 16-25 minute range, any longer and we lose the stimulus. Today is all aerobic capacity so the question becomes: how well can you breathe "comfortably" while you work? As best you can, take big breaths into your stomach, nice and controlled, and you'll be able to empty the tank BIG time on that last bike. Go get em'.